The shelf life of pyrite - effects of pyrite weathering in exposed continental shelves on global CO2 and O2

During times of low sea-levels in glacials there is evidence of large-scale pyrite oxidation on exposed continental shelves. While this process directly reduces atmospheric oxygen levels, acid drainage generated by this reaction increases the release of CO₂ through carbonate buffering within the previously marine shelf sediments. Although this scenario is expected to result in negative feedback, sea-level and atmospheric CO₂ levels have co-varied throughout most of the last 800 thousand years (ka) for which direct records of CO₂ exist. Only during peak glacial conditions with sea-levels as low as 125 m lower than today, CO₂ levels have reached an apparent lower limit around 190 ppm independent of decreasing sea-levels. Here we show that pyrite driven release of CO₂ and decline of O₂ during six of the last nine glacial-interglacial cycles are focussed in 10 ka to 40 ka-long periods preceding glacial terminations.

Using a sea-level driven model of pyrite weathering in drained continental shelves, we demonstrate that repeated sea-level low-stands force pyrite oxidation to ever greater depths. This occurs whenever the duration of an interglacial is insufficient to restock the shelf pyrite inventory through sulphate reduction in the shelf sediments. During the Quaternary, the decreasing amount of pyrite in the exposed shelf sediments represents a discharging 'acid capacitor' (Kölling et al., 2019). This model was inspired by experience from modelling pyrite weathering in open-pit lignite mine overburden material which may be interpreted as a scaled-down model of glacial continental shelf exposure during sea-level low-stands.

If pyrite oxidation forced CO₂ release specifically at low sea-levels was sufficient to amplify the orbitally driven climate forcing and trigger glacial terminations, the absence of CO₂ release caused by exposed pyrite rather than an astronomically controlled '100 ka pacing' might have extended the length of glacial-interglacial cycles from one to two or three obliquity cycles. Future ocean drilling specifically aiming to recover long cores on shelves could reveal the existence of a 'pyrite gap' that should exist between surficial young pyritic layers and deeper old pyritic sequences with indications of acid leaching.

 

Kölling, M., Bouimetarhan, I., Bowles, M.W. et al. Consistent CO₂ release by pyrite oxidation on continental shelves prior to glacial terminations. Nat. Geosci. 12, 929–934 (2019). https://doi.org/10.1038/s41561-019-0465-9